Historically, hydro and wind powers were among the first energy sources to be exploited by mankind. After a period of time when other forms of energy sources have cast a shadow on those, there is nowadays a renewed interest in energy conversion systems operable by renewable energy resources such as wind for instance.
Compared to hydropower generation, power generated by means of a windmill generally requires less civil works, depending on the method used to mount and anchor the windmill, and the impact on the environment is minimal.
Generally speaking, wind energy is used through two basic types of windmills. On the one hand, the vertical axis windmills are omni-directional, i.e. they are capable of reacting to the winds from any direction, and the power is typically available at the ground level. On the other hand, horizontal axis windmills make use of a rotating disk that must be rightly aligned at all times in relation to the wind direction.
Even though the basic configuration of vertical axis windmills is therefore simpler than that of its horizontal counterpart due to the above mentioned characteristics, there are features of vertical axis windmills that need to be optimized in order to obtain an efficiency as high as its potentiality is.
For one thing, the power potential is proportional to the air density multiplied by the swept area, i.e. the projected blade area, multiplied by the cube of the air velocity. In the case where the fluid is air, the density is weak, and thus the swept area is of importance. As far as the forces on the devices and on the supporting structure are concerned, the windmill has therefore to withstand important wind speeds.
In summary, the amount of wind energy captured by a windmill depends on the section of the air flux sweeping the blades. This section is defined as the projection of the area effectively swept onto a plane perpendicular to the airflow. The geometry of the swept section is a matter of design choice. In the prior art are found variable geometry designs and fixed geometry designs, depending on the technology employed (see FIGS. 1 to 3).
Considering the above, a problem to be solved lies in the requirement that the blades should provide a maximized swept area for a maximal wind catching ability, while simultaneously they must be able to withstand high bending moments and forces. It has to be considered that on the one hand wind rotors having blades of the most efficient wind catching structure cause the most stress forces on the frame for the wind rotor and in extreme situations may cause damage thereto, and that, on the other hand, blade configurations which cause the least stress to the windmill structure in turn are the least efficient in wind energy gathering ability and transfer.
Efforts have been made in the art so as to design efficient blades. For example, FIG. 1 shows a type of blades 10 and 12 directly connected to the ends of a vertical axis 14. In the case of a blade connected through wing-beams to a central shaft beams, FIG. 2 shows an example of a blade 16 having a curved profile, and FIG. 3 shows a blade 18 having a square profile.
Also, the costs of the required civil work and of erection have to be considered.
In spite of intense work in the field and numerous proposed arrangements to generate electricity from the wind, there is still room for improvement in the design of the overall structure of a windmill, of the blades, of the power unit configuration and in the method for erection thereof, so as to design a vertical axis windmill of high performance with a reduced potential for damaging of the overall structure, which can be efficiently and economically manufactured and erected.